Why did evolution leave beards on men?

This is good, I like it.
Best I can come up with is something to do with it having been advantageous for men to be able to conceal their faces, maybe making them better at negotiations when meeting other competing social groups, whilst they liked being able to more easily tell what women were thinking.

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If it's just a marker of sexual maturity it doesn't have to serve any particular practical purpose, like my earlier example of flanges in male orang utan. Flange flange flange.

Yes, it's only the dominant male develops them and another will when it's going to challenge. But the point about no practical purpose applies. Flange flange flange. Or think of the gaudy plumage on a lot of male birds etc.

This article indicates it isn't straightforward, but some evidence links beards to dominance and therefore increased reproductive success.

A man’s ability to grow a fulsome beard isn’t actually neatly linked to his testosterone levels. Despite this, a number of studies have suggested that both men and women perceive men with beards as older, stronger and more aggressive than others. And dominant men can get more mating opportunities by intimidating rivals to stand aside.

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But the issue is complex and can't really be distilled down into a single reason.

They do serve a superb practical function though - a face scarf. Your face is kept noticeably warmer in cold weather than when it is cleanly shaved or close-cropped.